Self-heating filter

ABSTRACT

In a roll of electrically conductive filter materials (21-28) coiled together with spacers (31) to be (38) disposed in alternate relation to the filter materials, the filter materials (21-28) have a plurality of pairs of adjoining edges joined together to define openings for admitting gases into the roll, so that the filter materials (21-28) may collect particulates from the gases flowing therethrough. A central electrode (41) is connected in the center of the roll, while a cylindrical outer electrode (42) is connected about the outer periphery of the roll, so that upon application of an electric voltage thereacross, the two electrodes may supply an electric current to the filter materials (21-28) to heat them for regeneration.

TECHNICAL FIELD

This invention relates to a self-heating filter, i.e. a filter of thetype which can heat itself, and more particularly, to a self-heatingfilter for removing particulates from exhaust gases of a diesel engine,or the like.

BACKGROUND ART

A known filter for collecting the particulates discharged by a dieselengine, or the like is disclosed in, for example, Japanese PatentApplication Laid-Open under No. 131518/1980 or 170516/1983.

This filter is made of a ceramic material, and is provided upstreamthereof with a regenerating heater for removing particulates by burningfrom the filter and thereby regenerating it.

There has also been proposed a filter provided with a regeneratingburner, instead of the heater, for blowing a flame against the filter toregenerate it.

There has also been proposed a self-heating filter comprising a ceramichoneycomb which is formed from electrically conductive silicon carbideand can heat itself electrically for regeneration by burningparticulates (Japanese Patent Application Laid-Open under No.110311/1982).

The filter mentioned above as a first example of the known devices has,however, been found to present a number of problems. It is necessary todetect accurately the amount of particulates collected by the filter. Ifthe amount is too small, fire goes out sooner than the filter can beregenerated, but if it is too large, the filter cracks or melts down.The filter is easily broken, since it is likely to have a temperaturedistribution lacking uniformity, and is also low in mechanical strength.

The filter mentioned as a second example requires the burner as specialequipment. Moreover, it presents the same problems of cracking, etc. asthe first example of filter does.

The self-heating filter mentioned as a third example has the advantagethat it can be regenerated without the aid of any special equipment,insofar as it can heat itself.

This filter is, however, difficult to manufacture, since silicon carbideshows a high degree of shrinkage when a molded product thereof is fired.Moreover, it is easily broken during its regeneration under heat. Forthese and other reasons, the filter has been available only in a limitedvariety of shapes, or with a great deal of difficulty in the selectionof an appropriate value of electrical resistance.

Under these circumstances, it is an object of this invention to providea self-heating filter which has a high degree of strength to withstandregeneration under heat and a high efficiency in the collection of fineparticles, and is easy to manufacture.

DISCLOSURE OF THE INVENTION

According to this invention, a self-heating filter comprises a roll ofan electrically conductive filter material having a plurality of pairsof adjoining edges joined together to define openings facing the flow ofgases to collect fine particles from the gases passing through thefilter material, and electrodes provided at each of the center and outerperiphery of the roll for applying an electric current to heat thefilter material.

The most outstanding feature of this invention resides in theelectrically conductive filter material coiled in a roll, and having aplurality of pairs of adjoining edges joined together to define openingsto collect fine particles from the gases passing through the filtermaterial. Another important feature resides in the electrodes providedin the center, and along the outer periphery, respectively, of the rollof the filter material.

The filter material is a gas-permeable porous material which can collectfine particles from gases. The material is also electrically conductive.The filter material may, for example, be a composite material formedfrom a metal net, and a porous sintered product of a metal or ceramicpowder, as will be described in Example 1. The metal powder may, forexample, be of a ferritic stainless steel containing aluminum.

The filter material is so joined as to define openings facing the flowof gases, and thereby causing them to pass through the filter materialbefore they are exhausted.

The roll of the filter material may consist of a single sheet ofmaterial, or, for example, two or 10 sheets of material coiled together.A spacer is preferably interposed between every two adjoining layers, orsheets of the rolled filter material for keeping them spaced apart fromeach other, and also for reinforcing the filter material. The filtermaterial may be a corrugated or plane sheet, but it is essential toensure that a clearance be maintained between every two adjoininglayoffs of the filter material for admitting and exhausting gases.

Likewise, the spacer may be a corrugated or plane sheet, but it isessential to ensure that gases be allowed to flow between the filtermaterial and the spacer.

The spacer may, for example, be formed from a ferritic stainless steelsheet containing aluminum. The spacer may have an insulating oxide layerformed on its surface by, for example, oxidation by heat. The rolledfilter material preferably has an inner end connected to the electrodein its center, and an outer end connected to the electrode along itsouter periphery, so that an electric current may be passed through thefilter material for heating it.

The spacer may, or may not be interposed between the adjoining edges ofthe rolled filter material which are joined together. If the joining ofthe edges is effected by welding, brazing, sintering, etc. and enables,therefore, electrical connection, too the electric current supplied forheating the filter material flows not only spirally along the filtermaterial, but also radially of the filter.

The edges of the filter material can also be joined together by using aninorganic insulating adhesive.

The self-heating filter of this invention exhibits a particularly goodresult in the collection of particulates from exhaust gases of a dieselengine, though it can also be used very effectively to remove fineparticles from other gases, such as smoke. It can also be used as asupport for a catalyst for purifying exhaust gases if a filter materialhaving a larger pore diameter is employed.

The fine particles which gases contain are collected on and in thefilter material when the gases pass through it. After the filter hasbeen used for a certain period of time, the flow of gases isinterrupted, and an electric current is applied between the center andouter periphery of the roll for heating the filter material, and air ispassed through the filter material for burning fine particles.

The electric current flows spirally through the filter material andthereby generates heat therein. The fine particles collected by thefilter material are burned by the heat and the air, and are furtherraised in temperature by the heat of combustion which the particlesthemselves produce, whereby they are removed from the filter.

As the electric current flows spirally along the rolled filter material,the filter of this invention can be heated substantially uniformly inboth its central and outer peripheral portions. Therefore, it has auniform temperature distribution which ensures that the filter materialnot be broken, or otherwise damaged. Its uniform heating also enables ahigh efficiency of regeneration.

The self-heating filter of this invention is easy to manufacture, sinceit can be manufactured by rolling the filter material. Thus, thisinvention provides a self-heating filter which has a high degree ofstrength to withstand regeneration under heat and a high efficiency inthe collection of fine particles, and is easy to manufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view of the self-heating filter whichwill,hereinafter be described as Example 1;

FIG. 2 is a front elevational view of the self-heating filter accordingto Example 1;

FIG. 3 is an overall explanatory view of the self-heating filteraccording to Example 1;

FIG. 4 is an enlarged fragmentary sectional view taken along the lineA--A of FIG. 1;

FIG. 5 is a sectional view taken along the line B--B of FIG. 4;

FIG. 6 is a fragmentary sectional view of the filter according toExample 1 which shows the junctions between filter materials andspacers;

FIG. 7 is a perspective view of a filter material in the filteraccording to Example 1;

FIG. 8 is an explanatory view of the filter material in the filteraccording to Example 1;

FIG. 9 is a fragmentary enlarged sectional view of the filter materialin the filter according to Example 1;

FIG. 10 is a view illustrating the direction in which an electriccurrent for heating the filter material flows across the filteraccording to Example 1;

FIG. 11 is a view illustrating the flow of the electric current acrossthe filter according to Example 1, as represented in front elevation;

FIG. 12 is a top plan view of a particulate collecting case equippedwith self-heating filters according to Example 1;

FIG. 13 is a sectional view taken along the line C--C of FIG. 12;

FIG. 14 is a graph showing the distribution of temperatures across thefilter according to Example 1;

FIG. 15 is a fragmentary sectional view of a self-heating filteraccording to Example 2;

FIG. 16 is a graph showing the distribution of temperatures across thefilter according to Example 2;

FIG. 17 is a view illustrating the direction in which an electriccurrent for heating the filter material flows across the filteraccording to Example 2, as represented in front elevation;

FIG. 18 is a fragmentary sectional view of a self-heating filteraccording to Example 3;

FIG. 19 is a fragmentary sectional view of a modified form of filteraccording to Example 3;

FIG. 26 is a fragmentary sectional view of another modified form offilter according to Example 3;

FIG. 21 is a fragmentary sectional view of a self-heating filteraccording to Example 4;

FIG. 22 is a sectional view of a self-heating filter according toExample 5, as taken along the line D--D of FIG. 23; and

FIG. 23 is a sectional view of a particulate collecting case equippedwith filters according to Example 5.

BEST MODE OF CARRYING OUT THE INVENTION [EXAMPLE 1]

A self-heating filter embodying this invention will now be describedwith reference to FIGS. 1 to 13. The filter comprises a roll of eightsheets 21 to 28 of an electrically conductive filter material coiledtogether with spacers 31 to 38 disposed in alternate relation to thefilter material, as shown in FIGS. 1 to 6. The filter materials 21 to 28are corrugated, while the spacers 31 to 38 are plane sheets (FIG. 5).

The filter materials 21 to 28 have a plurality of adjoining edges (e.g.211 and 221) joined together to define openings facing the flow ofgases, as shown in FIG. 4, so that the filter 1 may collect fineparticles from the gases passing through the filter materials 21 to 28.The filter materials 21 and 22, 23 and 24, 25 and 26, and 27 and 28 arerespectively shown together in FIG. 1, since each such pair of filtermaterials are joined together at the adjoining edges, as shown in FIG.4.

The spacers 31 to 38 are interposed in alternate relation to the filtermaterials 21 to 28. The position of every other spacer 32, 34, 36 or 38is shown by a broken line in FIG. 1, while the position of each of theother spacers 31, 33, 35 and 37 cannot be shown by a broken line, but isindicated by its parenthesized reference numeral, since it is joinedtogether with the adjoining filter materials, as shown in FIG. 6.

The roll of the filter materials 21 to 28 has its center fastened to acentral electrode 41,1and its outer periphery fastened to a cylindricalouter electrode 42, as shown in FIGS. 2 and 3. The outer electrode 42serves also as a metal frame for holding the roll of the filtermaterials 21 to 28 and the spacers 31 to 38 together in position. Thecentral and outer electrodes 41 and 42 form an electric circuit with apower source 43 and a switch 44 for supplying an electric current forheating the filter materials, as shown in FIG. 3.

The spacers 31 to 38 keep every adjoining two of the filter materials 21to 28 in a spaced apart relation from each other. One of the spacers isinterposed between the edges of every two adjoining filter materialsjoined together. For example, the spacer 31 is interposed between oneedge 211 of the filter material 21 and one edge 221 of the filtermaterial 22, as shown in FIGS. 4 and 6, while the spacer 32 isinterposed between the other edge 222 of the filter material 22 and oneedge of the filter material 23. Thus, the filter materials 21 to 28 arejoined together along the adjoining edges to define openings facing theflow of gases which is shown by arrows in FIG. 4.

The corrugated filter materials 21 to 28 and the plane spacers 31 to 38are alternately coiled, as shown in FIG. 5. Each of the spacers 31 to 38has an insulating layer formed on its surface, as shown at 315 in FIG.6, so that the filter materials and the spacer therebetween areelectrically insulated from each other except at their edges joinedtogether. The insulating layer 315 on the spacer 31 is of Al₂ O₃, whichis employed to form an insulating layer on each of the other spacers 32to 38, too. The insulating layer 315 terminates inwardly of thejunctions between the filter materials 21 and 22 and the spacer 31 andthereby enables their electrical connection at the edges along whichthey are joined together.

Referring to the use of the filter 1, gases, such as the exhaust gasesof a diesel engine, are introduced into the filter 1, and passed throughthe porous filter materials 21 to 28, as shown by arrows in FIGS. 3, 4and 5, whereby the particulates, or fine particles which the gasescontain are collected by the filter materials 21 to 28.

The filter 1 can be regenerated if an electric voltage is applied acrossthe central and outer electrodes 41 and 42 through the circuit shown inFIG. 3. The application of the voltage results in the flow of anelectric current to the filter materials 21 to 28 and the spacers 31 to38 and the heating thereof, whereby the fine particles collected by thefilter 1 are removed by burning, and the filter 1 regenerated.

Description will now be made of the filter in further detail, along witha process for manufacturing it.

Each of the filter materials 21 to 28 has a length of about 360 mm, awidth of 20 mm and a thickness of about 0.25 mm, and its corrugationshave a pitch of about 2.5 mm and a height of about 1.5 mm. The followingdescription of the filter materials will mainly be made with referenceto only the filter material 21. The filter material 21 has a pair ofopposite edge portions 211 and 212 flattened in opposite directions, asshown in FIG. 7, and each having a width of about 3 mm. The filtermaterial 21 is a sintered metal product.

Description will now be made of a process for manufacturing the filtermaterial. The filter material 21 has as its skeleton a wire net 215 madeby cutting and stretching a sheet, and having a multiplicity of openings210, as shown in FIG. 8. The wire net 215 is corrugated by using gears,etc., and is flattened along the edges 211 and 212. The openings 210 arediamond-shaped, and have a long diameter of about 2 mm and a shortdiameter of about 1 mm. The net 215 is of ferritic stainless steelconsisting of iron, chromium and aluminum, and containing very smallamounts of other additives, and is, therefore, highly resistant to heat.

The filter material 21 is made by filling the wire net 215 with a metalpowder, as shown in FIGS. 8 and 9, and sintering it. The metal powderconsists mainly of iron, chromium and aluminum, and has an aluminumcontent of at least 5% by weight. It is a mixture prepared by mixingparticles having an average diameter of about 10 microns and particleshaving an average diameter of about 30 microns in a ratio of 1:3.

A slurry is prepared by mixing 100 parts by weight of the metal powderwith 0.3 to 5 parts by weight of a binder, such as methyl cellulose, and20 to 200 parts by weight of water. The wire net 215 is dipped in theslurry, or coated with it, so that the slurry may be deposited in thewire net.

After the slurry has thoroughly been dried, it is fired at a temperatureof 1000° C. to 1300° C. for 0.5 to 40 hours in a vacuum to form asintered mass 216 of the metal powder, whereby the wire net 215 havingits openings 210 filled permanently with the sintered mass 216 isobtained as the filter material 21. Then, the filter material 21 isheated at a temperature of 850° C. to 1100° C. for 2 to 10 hours in theopen air, whereby a film of Al₂ O₃ is formed on the surface of thefilter material 21.

FIG. 9 shows an enlarged section of the filter material 21. It shows aporous sintered body 216 of the metal powder filling the openings of thewire net 215 and covering its wire segments. The filter material 21collects fine particles on and within its surface from gases G passingthrough the pores of the sintered body 216.

The pore diameter of the filter material depends on various conditionsincluding the size and properties of the fine particles to be collected.It can be altered if the particle size and shape of the metal powderused as the starting material, the sintering conditions, etc. areappropriately selected.

The filter material 21 may consist solely of the sintered body 216 ofthe metal powder, but the use of the wire net 215 as a support makes itpossible to obtain a filter material having a desired shape and a higherdegree of strength. The wire net also functions to form a path for thestabilized flow of an electric current.

Each of the spacers 31 to 38 has a length of about 360 mm, a width of 20mm and a thickness of about 0.05 mm. Only the spacer 31 will be referredto in the following description of a process for manufacturing thespacers.

The spacer 31 is formed from a sheet of ferritic stainless steel havingthe same composition with that used for making the wire net 215 ashereinabove described. The stainless steel sheet is heated at atemperature of 850° C. to 1200° C. for one to 10 hours in the open air,whereby an oxidation-resistant film of alumina is formed on its surface.Then, its surface is coated with an alumina sol, and after it has beendried, it is heated at a temperature of 850° C. to 1200° C. for one to10 hours in the open air to form an alumina layer having a greaterthickness and defining an insulating layer 315, as shown in FIG. 6.

The spacer 31 formed from the stainless steel sheet can be heated by theelectric current applied across the central and outer electrodes. Thespacer can, however, be formed from a sheet of a ceramic, or otherheat-resistant and electrically insulating material, too. In that case,the spacer 31 is not electrically heated.

The central electrode 41 is a stainless steel rod having a diameter of 8mm and a length of 50 mm, and the outer electrode 42 is a cylindricalbody of stainless steel having an outside diameter of about 80 mm, alength of 20 mm and a wall thickness of about 1.5 mm.

Description will now be made of a method of assembling the filtermaterials 21 to 28, the spacers 31 to 38, and the central and outerelectrodes 41 and 42 into the filter 1.

The base ends of the eight filter materials 21 to 28 and the eightspacers 31 to 38 are joined to the central electrode 41, for example, bywelding or brazing, as shown in FIG. 1. Then, they are coiled togetherinto a roll, and the roll is inserted in the outer electrode 42 havingan inner surface coated with a brazing material, as shown in FIGS. 1 and2.

The assembly is heated at a temperature of 1000° C. to 1200° C. in abrazing furnace, whereby the roll is brazed to the outer electrode 42.The adjoining edges of the filter material 21 and the spacer 31 arejoined together, for example, by welding or brazing, as shown in FIGS. 4and 6, after the insulating layer has been removed from the spacer atthe junction. The other filter materials and spacers are joined togetherin the same way, whereby the filter 1 is obtained.

The filter 1 as assembled, except the central electrode 41, has anoutside diameter of about 80 mm and a length of 20 mm. These dimensionsdepend on the conditions under which the filter will be used, the valueof electrical resistance as required of the filter to produce heat forregeneration, etc.

Description will now be made of the application of an electric currentfor causing the filter to produce heat. If an electric voltage isapplied across the central and outer electrodes 41 and 42 through thecircuit shown in FIG. 3, an electric current flows through the filtermaterials 21 to 28 and the spacers 31 to 38. The current flows throughthe assembly both spirally and radially as shown by arrows and brokenlines U and arrows and solid lines R, respectively, in FIG. 11. Theradial flow of the current occurs through the edges of the filtermaterials 21 to 28 which are joined to one another, as shown in FIG. 10,while the spiral flow occurs through the filter materials 21 to 28 fromthe central to the outer electrode.

If the electric current flows only spirally, the filter has a uniformcurrent density and produces heat uniformly. If it flows radially, too,the filter has a somewhat higher current density and therefore asomewhat higher temperature in its central portion. Therefore, it isdesirable to raise the radial electrical resistance of the filter tosome extent, for example, by elongating a path for the radial flow ofthe electric current, so that it may have a uniform current density.

The filter produces heat uniformly if the edges of the filter materialsare not joined by welding, brazing, etc. The joining of the adjoiningedges as hereinabove described, however, has the advantage of ensuringthat the filter materials be joined together effectively for thecollection of fine particles.

If the filter is used in a vibrating machine or structure, such as anautomobile, there is every likelihood that the adjoining filtermaterials and spacers may be displaced from each other and protrude fromthe filter, i.e. the problem of the so-called scoping is very likely tooccur, if the edges are not joined together. This problem can be avoidedif the edges are joined as hereinabove described. It is also possible toavoid problems of e.g. thermal stress which would otherwise result fromthe joining of different kinds of materials.

Attention is now directed to FIGS. 12 and 13 showing by way of examplethe use of the filter 1 for removing particulates from the exhaust gasesof a diesel engine. The apparatus shown in FIGS. 12 and 13 includeseight filters 1 installed in a case 5 in parallel to the flow of exhaustgases G. The case 5 has a gas admitting passage 51 and a gas exhaustingpassage 52 between which the filters 1 are installed. The centralelectrodes 41 project upwardly from the filters 1 and are electricallyconnected to the power source 43. An insulating material 54 isinterposed between the case 5 and each electrode 41.

The exhaust gases G enter the case 5 through its gas admitting passage51, enter the filters 1 through openings 53, and leave the filters 1into the gas exhausting passage 52, as shown by arrows in FIG. 13,whereby the particulates which the gases G contain are collected by thefilter materials. More specifically, there was achieved a particulateremoval percentage of about 75% as measured by a smoke meter when adiesel engine having a displacement of 3.4 liters was operated at arotating speed of 1045 rpm and a load of 200 Nm.

Reference is now made to FIG. 14 showing the distribution oftemperatures as observed across one of the filters 1 in the case 5 whenit was electrically caused to produce heat. FIG. 14 shows thetemperatures of the filter as measured at a number of points along itsradius of 38.5 mm starting at its central electrode 41 which is shown at0. These results were obtained when a voltage of 4.5 V was appliedacross the filter 1, while air was passed through the filter 1 at a rateof four liters per minute. As is obvious from the graph, the filtershowed a considerable temperature drop toward the outer electrode.

An attempt was made to remove the particulates from the filter byburning, while supplying air as hereinabove mentioned, to therebyregenerate it. The air was supplied to flow in the same direction as thegases G. A regenerating efficiency of about 75% was achieved. The filterwas heated 20 times repeatedly, but was not damaged at all.

As is obvious from the foregoing, the filter embodying this inventionhas a high degree of strength to withstand regeneration under heat and ahigh efficiency in the collection of particulates, and is easy tomanufacture.

[EXAMPLE 2]

The filter according to this example which is shown in FIG. 15 issubstantially identical in construction to that of Example 1, but isdistinguished from it by the edges of the filter materials which arejoined together by an electrically insulating adhesive. The filterincludes the same filter materials 21 to 28, and spacers 31 to 38 as thefilter of Example 1 does. The edges 211 and 221 of the filter materials21 and 22, and the spacer 31 interposed therebetween, for example, are,however, joined together at their butt ends by the electricallyinsulating adhesive 61, as shown in FIG. 15.

Preferred examples of the adhesive are of a ceramic material, such asAl₂ O₃ or SiO₂. The adhesive preferably has a coefficient of thermalexpansion which is close to that of the filter materials and spacers.The other adjoining edges of the filter materials and spacers arelikewise joined together.

According to another important feature of the filter under description,the whole spacer 31, for example, including its edge interposed betweenthe filter materials, is covered with the insulating layer 315, asopposed to Example 1 (FIG. 6). Therefore, the edges 211 and 221 of thefilter materials 21 and 22 are electrically insulated from each other bythe insulating layers 315 on the spacer 31.

An electric current was applied to the filter as hereinabove described.FIG. 16 shows the distribution of temperatures as observed across thefilter, and FIG. 17 shows the direction in which the current was foundto flow through the filter.

As is obvious from FIG. 17, the electric current flowed only spirally asshown by arrows and broken lines U, insofar as the filter materials 21to 28 are electrically insulated at their joined edges by theelectrically insulating adhesive 61 and the insulating layers 315, etc.on the spacers.

The filter, therefore, has a uniform current density and can heat itselfvery uniformly with a smaller difference in temperature between itscentral and outer peripheral portions, as is obvious from the comparisonof FIG. 16 with FIG. 14 (Example 1). Thus, a regenerating efficiency ofabout 87%, which was higher than what was obtained in Example 1, couldbe achieved upon regeneration by repeating the procedures employed inExample 1. In the other aspects of its use and performance, the filterunder description is comparable to that of Example 1.

The regeneration of the filter as hereinabove described relies solelyupon heat, i.e. the heat produced in the filter materials by an electriccurrent, and the heat produced by the combustion of particulates. Theregeneration may, however, further include the passing of air into thefilter in the opposite direction to the flow of the gases to bepurified, or from downstream, while relying upon the heat. The air issupplied after the filter has been heated to or above the combustiontemperature of the particulates, and when the particulates have startedburning in their layers contacting the filter materials. In other words,the air is supplied to blow the particulates away from the filtermaterials when not all of the particulates have burned as yet. Theparticulates which have been blown out of the filter are collected in avessel positioned upstream of the filter to be burned, or otherwisedisposed of. This is the advantage of the filter of the self-heatingtype which cannot be achieved if only the supply of air as hereinabovedescribed is relied upon for regeneration, while no heat is relied upon.The mere use of air can hardly regenerate the filter perfectly, sincethe particulates adhere strongly to the filter materials.

Although the filter materials 21 to 28 in the filter as hereinabovedescribed are supplied with an electric current independently of oneanother, it is alternately possible to construct the filter so that eachpair of filter materials, such as 21 and 22, or 23 and 24, may form asingle unit, as far as the supply of an electric current is concerned.

An attempt was made to study the effect which the external dimensions ofthe filter might have on the amount of electricity as required forburning particulates. A comparative filter was prepared by increasingeach of the width of the filter materials 21 to 28 and the spacers 31 to38 and the lengths of the central and outer electrodes 41 and 42, i.e.the comparative filter, except the central electrode, had an outsidediameter of about 80 mm and a length of 40 mm. The comparative filterhaving a heat capacity twice larger than that of the filter ashereinabove described required about twice as large an amount ofelectricity for heating to the combustion temperature of particulates,if the same heating rate was employed. It, however, required onlysubstantially the same amount of electricity for maintaining thecombustion temperature of the particulates, as what the filter ashereinabove described did. This is due to the fact that the comparativefilter had only 1.3 times as large an external surface area despite itstwice larger volume, and did, therefore, not lose a very large amount ofheat by radiation. The comparative filter showed a regenerationefficiency of 86%.

It is, however, desirable to select the size, shape and number of thefilters employed appropriately in accordance with the amount ofparticulates to be removed, and the amount of electricity available, asthe amount of electricity required for burning the particulates and itscontrol depend on the size and shape of the filters.

[EXAMPLE 3]

Reference is now made to FIGS. 18 to 20 showing modified forms of filtermaterials and spacers. FIG. 18 shows plane filter materials 21 to 23 andcorrugated spacers 31 and 32, as opposed to their counterparts in thefilter according to Example 1. The other filter materials and spacersnot shown are also shaped like those shown in FIG. 18. Moreover, thefilter materials and spacers may both be corrugated, though not shown,if they are appropriately spaced apart from each other. In any otheraspect of its construction and use, the filter shown fragmentarily inFIG. 18 is identical to what has been described as Example 1.

FIG. 19 shows fragmentarily a modified form of filter according toExample 1 in which the spacers are each in the form of a wire net asshown at 36. FIG. 20 shows fragmentarily a modified form of filteraccording to Example 1 which includes spacers 36 of the type shown inFIG. 19 in alternate relation with plane spacers 31 and 33.

Although the filter according to Example 1 has eight sheets each offilter materials and spacers, it, as well as any modified form thereof,may alternatively be constructed with a different number of filtermaterials and spacers.

[EXAMPLE 4]

FIG. 21 shows a modified form of filter according to Example 1 in whicheach pair of adjoining edges of filter materials, e.g. the edges 211 and221 of the filter materials 21 and 22, are joined together directlywithout having the edge of a spacer, e.g. 31, interposed therebetween.The spacer 31 terminates inwardly of the edges 211 and 221. Compare FIG.21 with FIG. 4 showing Example 1. The electric current flows bothspirally and radially through the filter, as is the case with Example 1,and as shown in FIG. 11.

Although the filter materials in the filter of Example 1 have beendescribed as each comprising a sintered metal product coated with anoxide film, it is further possible to form a highly insulating layer onthe oxide film, for example, by flame spraying from a ceramic material,such as Al₂ O₃. In that case, no insulating layer need be formed on anyspacer, but if each pair of adjoining edges of filter materials and aspacer interposed therebetween are joined together, an electric currentflows spirally through the filter and heats it uniformly. The insulatinglayer on the filter material can be formed from, for example, a powderof a ceramic material such as Al₂ O₃ or SiO₂, or its fibers.

[EXAMPLE 5]

Attention is directed to FIGS. 22 and 23 showing an apparatus forremoving particulates from the exhaust gases of a diesel engine. Theapparatus comprises a casing 50 defining two chambers separated fromeach other, i.e. the left and right chambers 561 and 562. The casing 50has a gas admitting passage 560 and a gas exhausting passage 570, and isprovided between the gas admitting passage 560 and the two chambers 561and 562 with a damper 565 which is rotatable to open the inlet of one ofthe chambers for admitting the exhaust gases thereinto.

Each chamber 561 or 562 houses a plurality of filters 1, which havealready been described in detail, and the central electrodes 41 of thefilters 1 are connected in series to one-another. The filters 1 in eachchamber are joined together in a staggered fashion to form an array inwhich a first pair of filters 1 are joined together on one side, while asecond pair of filters 1 including one of the first pair of filters 1are joined together on the other side, so that the exhaust gases maygenerally enter the filters 1 on one side of the array, and leave themon the other side thereof, as shown by arrows in FIG. 23. The exhaustgases arriving from the engine enter the casing 50 through its gasadmitting passage 560, flow through the filters 1 in one of the chambers561 and 562 depending on the position of the damper 565, and leave thecasing 50 through its gas exhausting passage 570. The two chambers 561and 562 are separated from each other by a wall 35 (FIG. 22). Anelectric cable 421 (FIG. 22) is connected to the outer electrodes 42 ofthe filters 1.

The apparatus is useful for the continuous removal of particulates, asthe filters in one of the chambers are used for collecting theparticulates, while those in the other chamber closed by the damper 565are being regenerated.

INDUSTRIAL APPLICABILITY

As is evident from the foregoing, the self-heating filter of thisinvention is useful for collecting particulates from the exhaust gasesof a diesel engine, or the like.

What is claimed is:
 1. A self-heating filter comprising:a spiral roll ofan electrically conductive filter material having a plurality of pairsof adjoining edges joined together to define openings facing the flow ofgases to collect fine particulates from said gases passing through saidfilter material; and electrodes provided at each of the center and outerperiphery of said roll for applying an electric current to heat saidfilter material in such a manner that the electric current spirallyflows along said roll of the filter material.
 2. A filter according toclaim 1, wherein said filter material comprises a corrugated sheet.
 3. Afilter according to claim 1, wherein said filter material iselectrically insulated from an adjoining filter material.
 4. A filteraccording to claim 1, wherein said filter material is electricallyinsulated from an adjoining filter material except at adjoining edgesthereof.
 5. A filter according to claim 1, further comprising a spacerdisposed in alternate relation to said filter material for insulatingtwo adjoining filter materials.
 6. A filter according to claim 5 whereinsaid spacer comprises a perforated sheet in the form of a wire net.
 7. Afilter according to claim 5, wherein said filter material is formed froma metal net having a mesh portion on which a plurality of holes areequally distributed therein, and, a porous sintered body obtained frommetallic powder having the same composition as said metal net, saidporous sintered body covering said metal net.
 8. A filter according toclaim 5, wherein said filter material comprises a corrugated sheet andsaid spacer comprises a plane sheet.
 9. A filter according to claim 8,wherein said spacer provides electric insulation.
 10. A filter accordingto claim 9, wherein said spacer is a metal plate having an insulatedsurface.
 11. A self heating filter comprising:a center electrodedisposed at the center of the filter; an electrically conductive filtermaterial of which a first end is connected to said center electrode andis spirally coiled around said central electrode; a spacer having aninsulator thereon and disposed in alternate relation to said filtermaterial so that said spacer is spirally coiled around said centerelectrode in such a manner to be sandwiched by adjoining filtermaterial; and an outer electrode disposed on the outer periphery of saidfilter material, wherein a plurality of pairs of adjoining edges of saidfilter material is joined together to define openings facing a flow ofgases to collect fine particulates from gas passing though said filtermaterial and an electric current flowing from one of said electrodesflow spirally along said filter material.
 12. A filter according toclaim 11, wherein said electrically conductive filter material has anupper end and a lower end which are perpendicular to said first end,every other pair of adjoining said electrically conductive filtermaterials is joined together with said spacer interposed between saidpair of adjoining filter materials at said upper end thereof, and everyother pair of adjoining said electrically conductive filter materialswhich is different from said every other adjoining filter materials isjoined together with said spacer interposed between said pair ofadjoining filter materials at said lower end thereof so that a pair ofadjoining said electrically conductive filter materials defines anopening facing the flow of gases at said upper end thereof and a closureat said lower end thereof.
 13. A filter according to claim 11, whereinsaid filter material is formed from a metal net having a mesh portion onwhich a plurality of holes are equally distributed therein, and, aporous sintered body obtained from metallic powder having the samecomposition as said metal net, said porous sintered body covering saidmetal net, said spacer providing electric insulation.